nochmal der Bezug zum "Twist Off":
hier der Hauptlink:
http://www.yellowdefender.com/twist_off_1999/twist_off_result/index.htmauch schon über 10a alt. Der "wissenschaftliche" Ansatz behagt mir immer noch.
7.0 Discussion
Introduction
The field of rock crawling is in its early stages and we have a way to go before we know what modifications works best for our vehicles. However, tests like this one provide quantitative information to accelerate our progress. To my knowledge, this test is unique in that we are using fairly rigorous methods to quantify the correlation between observable behavior on the ramp and off-road performance. We were fortunate in that we were able to call on the help of very experienced D90 drivers to assess the changes in the on-road performance of D90s due to these modifications.
The modifications to the 6 fully tested vehicles represented a much broader range of approaches than I anticipated prior to the test. Because of this range, I feel that the data is adequate to support some generalizations on suspension design.
In this section, I will present some engineering observations on the performance of the various approaches. My hope is that these observations will promote discussions within (and outside) the D90 list concerning future developments in suspension designs for rock crawling vehicles. The discussion will be clearly biased toward the off-road performance (at rock crawling speeds) since the D90 list was originally created for the more hard-core off-road D90 driver.
RTI
The quantitative analysis presented in the previous section indicated that there was not a strong correlation between maximum RTI and the trail scores. Since competitors were allowed to use lockers, stability and clearance increased in importance relative to traction. The analysis did show a strong correlation between trail performance and suspension balance (i.e., the front and back axles build articulation equally).
Suspension Balance
To understand the effect of suspension balance, one can compare two hypothetical vehicles. Assume a vehicle with an unbalanced suspension generates all of its articulation from the rear axle and none from the front. Assume a second vehicle with a balanced suspension generates 50% of its articulation from the front and 50% from the rear. Now consider what happens when these two vehicles transverse a section that requires climbing a rock on the right side on otherwise flat ground. Also assume that both vehicles have sufficient articulation that all 4 tires remain on the ground. The unbalanced vehicle with the rigid front end will generate approximately twice the body tilt when its front tire climbs the rock than the vehicle with the balanced suspension. This is because the front axle of the unbalanced vehicle does not articulate relative to the body. When the front wheel drops off and the rear climbs the rock, the unbalanced vehicle experiences no body roll whereas the balanced vehicle experiences the same body roll as when its front tire was on the rock. Generally both vehicles will be able to transverse the rock on flat ground since the rock is within the articulation limits of the vehicles (i.e., all wheels remain on the ground). If this rock is part of an off-camber section that already has a vehicle half way towards its limits of tip over stability, then the additional roll angle caused by a rock can lead to tip over. In such cases, a balanced vehicle will generate less additional roll, allowing it to transverse a significantly taller rock without tipping over since it generates less maximum roll angle during the maneuver. Compounding this is the effect of the angle of the trailing arm of the rear axle.
Trailing Arm Angle
There are several forces that act between each side of the axle and the chassis. These are the spring/shock forces and the forces associated with the trailing arm. All of these forces act along the components (along the spring, along the shock, and along the trailing arm). If the trailing arm is not parallel to the ground, then there will be a vertical force component on the chassis due to this angle. When under power, the arm will be in compression. If the arm is sloped upward (i.e., the wheel is drooping), the force vector will have an upward component. This will cause that side of the vehicle to lift somewhat. With the additional lift, the wheel can drive under the vehicle, increasing the angle more, further increasing the upward forces on the chassis. While all of this is going on, the cg height of the vehicle is increasing. Normally these effects are not significant. However, if a vehicle is climbing an obstacle on a side slope, the vehicle may already be near the limits of its roll over angle. Under these conditions, it does not take much force along the trailing arm to further tilt the vehicle. Eventually, one gets to the point where it feels that all of the drive torque is going into tilting the vehicle and none is going into moving it forward. At this point, the only option is to back down and try a different line (or for the really brave, back down and use some momentum).
How can these upward chassis forces be reduced? One approach is to allow less wheel droop (and hence, less articulation) on the back (OME, SG2) so that the rear wheel cannot climb under the vehicle as much. This can be accomplished either through retained rear springs of sufficient stiffness, or limited axle movement due to shorter shocks. We can also use longer rear arms as this reduces the angle of the trailing arm relative to the ground, reducing the upward component of force on the chassis.
Another approach that sometimes works (this was the case on the last obstacle on Section 2) is to build in more articulation in the front. This will result in less body roll to begin with (SG1 and SG2), reducing the angle of the trailing arm.
How Much Articulation is Too Much?
The answer to this question depends on the trail and the vehicle. When lockers are not used, articulation can have a very large positive effect on traction. However, the use of lockers tends to lessen the traction advantages of articulation. What articulation does provide, when lockers are used, is an improved ability of a wheel to climb a rock. This is because the traction force on the rock, required for the climbing wheel, is less due to the easier upward movement of the wheel, and because the remaining wheels can push the vehicle harder into the rock, generating more traction force on the climbing wheel. Articulation improves off-road performance if there is not so much of it as to negatively effect the stability of the vehicle. Vehicles that spend most of their time in canyon bottoms (such as 21 Road) can probably afford more articulation than those that spend much of their time on off-cambered trails and steep articulated climbs. Vehicles with longer trailing arms, lower cgs, and wider tracks (i.e., buggies) can also possess more articulation. Vehicles with balanced articulation front and back can also afford more total articulation than those that generate most of their articulation from one axle.
Lift
The most direct and important effect of suspension lift is the lift raises the cg height, decreasing the lateral acceleration (or tip angle) at which the vehicle rolls. The high cg (relative to the track), combined with the short wheel base, is the primary reason for rollovers of sport utility vehicles on the road.
Another effect of the suspension lift is it changes the kinematics of the suspension. Raising the front can increase the bump steer because the drag link is no longer parallel to the ground, and decreases dynamic stability because caster is deceased. Most people understand the effect of these changes. A more esoteric change is the possible change in height of the roll centers for each axle. A roll center of an axle is the point that the body appears to be rolling about, in the same vertical plane as the axle. Standard practice is to design the suspension such that the rear axle roll center is higher than it is in the front because this improves straight-line stability of the vehicle. When a D90 is lifted, the rear axle roll center stays the same height (i.e., at the center ball joint), but the front roll center is raised because the center of the Panhard rod also raises. The result is a decrease in directional stability, resulting a bit more darting about on the road in response to road irregularities.
A secondary roll-center related effect is as the chassis is lifted, the average roll center height is raised, but not as much as the cg height. This results in a longer vertical lever arm between the cg and the roll centers. The driver feels this as additional body-roll in corners and off-cambers.
Off road, lifts provide 1) additional body and chassis clearance through boulder fields, 2) the ability to use larger tires to improve differential clearance and increase traction, and 3) increased approach, departure, and break-over angles. Another advantage is one can generate more upward wheel travel before hitting the wheel wells, allowing more articulation without further increasing the cg height of the vehicles due to articulation. There are several disadvantages of large lifts on off-road performance. One is the increased tendency to produce body roll because of the longer vertical lever arm between the cg and the roll centers. One can control dynamic body roll through stiffer shock valving, and dynamic and static body roll through stiffer springs. Unfortunately, stiffer springs and shocks reduce ride comfort, and stiffer springs reduce articulation. Other more serious disadvantages of large lifts are the decreased angles at which the vehicle is stable and the increased weight transfer to the down slope wheels. Vehicles tend to climb better if they can keep as much weight as possible on the front tires. Finally, lifting a vehicle increases the angles in the trailing arms, which leads to the jacking effect discussed earlier.
The 3-Link Front Suspension
Is the 3-link suspension a good thing? A clear advantage of the 3-link front suspension is the design does allow articulation to be increased in the front. This in-turn, allows the articulation to be more balanced front and back which can increase vehicle stability when rock crawling. If designed properly, such a design can also lead to more precision in handling because one can now use much less flexible bushings in the radius/torque arms at the axle.
However, there are downsides to the 3-link design from an engineering point of view:
1. It is difficult to find room for the third link because of the engine. SG’s approach is to put the 3rd link below the two radius arm links.
2. Under hard braking, a third link located below the two radius arms must withstand approximately 3.3 times as much force along its length as "both" the radius/torque arms on the stock design (depending on the diameter of the tires and the vertical separation in the axle mounting points). This is 6.6 times as much axial force as each stock link sees! Since this is a compressive force, a damaged lower third link can easily be buckled (we see this happen to the rear trailing arms under much lower loads than will typically occur at the front axle during braking).
3. These larger forces must to be carried by the axle mounts and the chassis mounts. The Rover uses well-trussed frame mounting points to support the lesser forces of the stock radius arms.
4. Because the forces are significantly higher, fatigue failure is more likely unless everything is properly designed.
5. The larger forces of the 3-link require stiffer bushings in the 3-links to maintain handling. However, stiffer bushings also transmit more shock loads to the chassis. This will accelerate fatigue effects relative to the stock design and decrease the overall shock loads that the design can withstand.
6. Finally, the roll resistance generated by the bushing in the stock front axle acts as an anti-roll bar, especially at larger body roll angles. This has a stabilizing effect on the road. A 3-link suspension will not have this stabilizing effect and one should either use stiffer springs up front than in the back, or use a detachable anti-roll bar up front to maintain the desired understeer characteristics of the vehicle while on the road.
A well-designed 3-link front suspension has good potential to improve the performance of the D90 off road. There is lots of room available in the front wheel wells of the D90 for upward movement. A design to utilize this room during articulation could significantly improve the performance of the D90 without increasing the cg height of the vehicle during articulation. However, the 3-link suspension must be carefully developed, due to the increased loads, and thoroughly tested under very harsh conditions before such a system can be considered commercially viable.
